It is known that ceramic films or membranes may be made of metal oxide materials, such as titanium dioxide. Such membranes are typically made by a sol-gel process in which a metal oxide precurser, typically an organometallic compound such as a metal alkoxide, is dissolved in an alcohol at low temperature and hydrolyzed and peptized to create a colloidal suspension or sol. Such sols can be slowly dewatered or can be coated onto substrates to form gels, which can be sintered into ceramic membranes, either unsupported or supported. By controlling the conditions of the sol-gel process, the metal oxide can be manipulated to form particles of selected size, which when fused into a particulate membrane, results in a membrane of a selected average hole or pore size. Normally in such membranes, porosity and conductivity are inversely related. This relation would be expected since increasing porosity is related to less contact between the sintered particles, and thus less surface area for electron flow. This property can, however, be a disadvantage in any application in which both high porosity and high conductivity are desirable.
One application in which both high porosity and high conductivity are desirable is the use of such a metal ceramic membrane as an electrode in an electrochemical cell. In such an application, it is desirable to have maximum surface area contact between the electrodes and the ionic solution of the cell. Higher porosity gives a higher effective surface area between the membrane used as an electrode and the solution into which it is placed. At the same time, it is clear that high conductivity is desirable for any material to be used as an electrode, to facilitate current flow into and out of the electrode from the appropriate electric circuitry.
It has been suggested previously that it is possible to dope other metallic materials into metal oxide ceramic membranes. However, most of the techniques previously suggested for the fabrication of doped membranes cannot be applied at as low a temperature as is required for the sol-gel process.